Curing and drying of plates for negative and positive plates had been investigated started in second half of the 20th century. By use of x-ray scattering tri basic and tetra basic lead sulfate crystal structures had been detected. Tribasic lead sulfate will be obtained at temperatures T < 60°C. In case of increased temperature tetra basic lead sulfate occurs and tri basic lead sulfate is converted into tetra basic lead sulfate in case of moisture or steam. The conversion rate depends on the moisture content and mainly on the temperature.
The structure as well as the performance of the batteries is different for the two different crystal structures.
Tri basic lead sulfate:
- The crystals of tribasic lead sulfate nucleate at temperatures below 60°C. The nucleation rate follows Arrhenius law (10°C doubles the reaction speed) and takes in average more than 12 h due to the low process temperature. Because of the side effect of free lead oxidation head and dry out of the plates occurs especially for reactive ball mill leady oxide. As a result the nucleation is threatened to be aborted.
- A complete and well networked crystal structure is imported to keep the porosity during operation of the batteries. Due to the low critical crystal size a huge number of spontaneous seeding crystal occur. Therefore the crystal size of tri basic lead sulfate is < 3 µm. Small crystals give rise for mass shading in case of deep discharge of the positive and negative plates. Therefore tri basic lead sulfate is not favored in deep discharge applications becoming more and more of importance in mild hybrid and start stop applications.
- In tribasic cured and dried plates for lead acid batteries the average pore size is about 0.4 µm. At high rate discharge the pores on the plate surface will be blocked by lead sulfate forming.
- The advantage of tri basic lead sulfate is the easy formation during production and good recharge in normal use of the batteries due to a huge BET surface.
Tetra basic lead sulfate:
- Compared to tri basic lead sulfate the critical nucleus size is huge. Therefore seeding crystals will occur only at higher temperature and low in number. As a result the tetra basic lead sulfate crystals become huge (30 - 70 µm). Due to the higher temperature being necessary the seeding crystals grow fast. Normally the nucleation to tetra basic lead sulfate will be carried out in steam at T > 80°C. This prevents dry out of the paste and the average time without addition of seeding crystals takes about 3 hours.
- The tetra basic lead sulfate crystals achieve 30 - 70 µm sizes (20 - 30 µm by adding 20 - 30% of red lead). This crystal structure is well with respect to discharge to deep discharge applications. But the efficiency of the active material is low due to the low BET surface.
- The pore diameter is about 1 to 2 µm depending on the crystal size.
- Tetra basic lead sulfate crystals of huge size need a long time and energy inefficient formation during production of the batteries. The recharge is less easy due to the low BET surface.
Our new developments:
We did develop new processes and technologies to manage the size of tetra basic lead sulfate crystals. We are able to engineer the crystal sizes according to the demands for the different applications. The different methods are described in in more details in the sub menus of this item.